| Modern living often leads to people eating when they have time to fit it in alongside work and a hectic social life. Maybe just a sandwich at your desk in the office or a burger on your way to the pub. But if you're going to eat properly (or maybe 'traditionally' would be a better term) then your meals will be made up of courses. On a simple level these are likely to include an appetiser or starter, followed by the main course and then a dessert.
The main course is the part of the meal that is intended to really fill you up. It will usually consist of the most substantial food and be the largest section of the meal.
You may also hear people refer to the main course in other business, as an analogy to the main course of a meal. Imagine a meeting where many minor points have been covered, someone could then say, "Let's get on to the main course." suggesting that it's time to deal with the more important issue on the agenda. | 
